1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device and, more particularly, to the method for manufacturing the semiconductor device having a capacitor as an information storing capacitive element in which a lower electrode is made of a metal that has a catalytic action such as ruthenium (Ru) or a like.
The present application claims priority of Japanese Patent Application No. 2002-220468 filed on Jul. 29, 2002, which is hereby incorporated by reference.
2. Description of the Related Art
Large Scale Integrations (LSIs) known as a representative of semiconductor devices are roughly classified into memory products and logic products, a former of which in particular has been developed remarkably with an up-growth of recent technologies for manufacturing the semiconductor devices. The memory products are further classified into Dynamic Random Access Memories (DRAMs) and Static Random Access Memories (SRAMs), most of which are made up of a Metal Oxide Semiconductor (MOS) transistor excellent in integration density. The DRAMs, in particular, are advantageous over the SRAM in utilization of a higher integration density and so can be reduced in costs, thus being widely used as memories in various kinds of storages in an information processing device such as a personal digital assistant, a personal computer, and a like.
In the DRAM, one memory cell is made up of a memory-cell selecting transistor, that is, a MOS transistor to be operated as a switch and a capacitor connected to the memory-cell selecting transistor so that information may be stored according to whether the capacitor is charged or not. It is to be noted that as an amount of information to be stored increases with a recent advance of an information-oriented society, an area occupied by the capacitor formed on a semiconductor substrate is decreased, so that a capacitance of the capacitor in each of the memory cells needs to be increased. If the capacitor does not have a capacitance large enough to store information, the memory cell readily malfunctions due to an external noise signal or a like, thus readily encountering various errors represented by a soft error.
Conventionally, as a capacitive insulating film of a DRAM capacitor, there has been used a metal oxide film or a like such as a silicon dioxide (SiO2) film, a silicon nitride (SiN) film, or a tantalum oxide (Ta2O5) film. Of these capacitive insulating films, the tantalum oxide film, in particular, is a metal oxide film and has a large dielectric constant as compared to the silicon dioxide film, the silicon nitride film, and so can be used as a capacitive insulating film to make up a capacitor having a large capacitance. Such the tantalum oxide film can be formed typically by Chemical Vapor Deposition (CVD), whereby it is easy to form the film. Further, to increase the capacitance of a capacitor, its lower electrode may be shaped in a form of a three-dimensional structure such as a cylinder in some cases.
Further, in a case where a capacitor is made by forming on the lower electrode a capacitive insulating film having a large dielectric constant such as the tantalum oxide films, such metal as ruthenium is used so that if a surface of the lower electrode is oxidized its oxide film may still have conductivity to thus prevent a decrease in capacitance and further, a Metal Insulator Metal (MIM)-type capacitor is used in which metal such as ruthenium is employed as a material of its upper electrode. For example, Japanese Patent Application Laid-open No. 2001-313379 discloses a method for manufacturing a semiconductor device in which a lower electrode is shaped in a form of a three-dimensional structure such as a cylinder and also a MIM-type capacitor is employed, as described above. The following will describe this semiconductor device manufacturing method along its steps with reference to FIG. 11A-11C.
First, as shown in FIG. 11A, for example, on a P-type semiconductor substrate 121, a memory cell transistor 126 made up of an MOS transistor is formed beforehand and a capacitor contact 129 is formed in a contact hole 128 formed in a silicon oxide film 127 and then a plasma oxidized silicon film 131 is formed throughout a surface via a plasma oxy-nitridized silicon film 130. Next, in a cylindrical trench 132 formed in the plasma oxidized silicon film 131, a lower electrode 134 made of ruthenium is formed via a barrier film 133. Next, on the lower electrode 134, a selective growth film 135 made of ruthenium is formed. A reference numeral 122 indicates an element isolation region 122, a reference numeral 123 indicates gate oxide film 123, a reference numeral 124 indicates a gate electrode 124, and a reference numeral 125 indicates an N-type diffusion region 125.
Next, as shown in FIG. 11B, a capacitive insulating film 136 made up of a tantalum oxide film is formed throughout the surface by CVD. The tantalum oxide film is formed by introducing into a reaction chamber an oxygen gas (O2) and a tantalum compound, for example, a tantalum penta-ethoxide (Ta(OC2H5)5]: hereinafter may be referred simply to as PET) as a material gas. Next, as shown in FIG. 11C, an upper electrode film 137 made of ruthenium is formed and patterned to thus form an upper electrode, thereby forming a capacitor.
A semiconductor device manufacturing method similar to that of the above-described publication is disclosed in, for example, Japanese Patent Application Laid-open No. 2002-26273.
As shown in FIG. 12, after the semiconductor substrate is placed in the reaction chamber kept at a predetermined film formation temperature and a predetermined film formation pressure, at a time t100, a material gas obtained by atomizing and vaporizing PET, which is a tantalum compound, with a nitrogen (N2) carrier gas to vaporize it and an oxygen gas are introduced together into it by the respective predetermined amounts, to start forming a tantalum oxide film. At a time t200 after a predetermined lapse of time, introduction of the material gas and that of the oxygen gas are stopped together. In this case, the oxygen gas is used to fill in oxygen vacancies liable to occur during formation of tantalum oxide films and remove organic matter. Further, by a conventional semiconductor device manufacturing method using the above-described film formation sequence, the material gas is introduced continuously at a time, to form a capacitive insulating film having a finally required film thickness.
However, it is found that by the conventional semiconductor device manufacturing method, the tantalum oxide film grows abnormally depending on a shape of an underlying electrode, that is, the lower electrode 134. It is also found that especially, for example, in a case where the cylindrical trench 132 has a high aspect ratio or the plurality of cylindrical trenches 132 occupies a large area, a capacitive insulating film 136, if made up of the tantalum oxide film by the conventional semiconductor device manufacturing method, grows little in the cylindrical trench 132 but grows abnormally at its upper part during its formation process. Such an abnormal growth has not been observed in a case where the cylindrical trench 132 has a small aspect ratio or the lower electrode 134 has a flat shape instead of a cylindrical shape in construction.
Based on these, the present inventor has concluded that the tantalum oxide film readily grows abnormally owing to the following phenomenon.
If, for example, by the conventional semiconductor device manufacturing method shown in FIGS. 11A-11C, the lower electrode 134 made of ruthenium is formed in the cylindrical trench 132 and then the capacitive insulating film 136 made up of a tantalum oxide film is formed, ruthenium acts as a catalyst to accelerate decomposition of the material gas and hence grow the tantalum oxide film, of which growth is further accelerated by a byproduct containing components of oxygen, carbon, nitrogen, or a like produced by the decomposition. As the decomposition of the material gas goes on, the cylindrical trench 132 is gradually filled with the byproduct until the material gas cannot be supplied into the cylindrical trench 132 any more, whereupon the tantalum oxide film does not grow on a side wall and a bottom of the cylindrical trench 132. At the upper part of the cylindrical trench 132, on the other hand, the byproduct is formed a lot, thereby acting to cause the tantalum oxide film to grow explosively hence abnormally. This deteriorates step coverage drastically.
FIG. 13 is an illustration for schematically showing the capacitive insulating film 136 made up of the tantalum oxide film that has grown abnormally at the upper part of the cylindrical trench 132. If step coverage is deteriorated in such a manner, the upper electrode 137, when subsequently formed on the tantalum oxide film in order to complete a capacitor, short-circuits with the lower electrode 134 readily, thereby resulting in malfunctioning of the capacitor.